<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .container{
            width: 400px;
            height: 400px;   
            background-color: yellow; 
            /* display: flex; || grid
            align-items: center; 
            justify-content : center; */
            /* position: relative;  */
        }
        .box {
            width: 20px;
            height: 20px;
            background-color: black;
            /* position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%,-50%); */
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="box"></div>
    </div>
</body>
</html>